1. Field of the Invention
The invention is generally related to the construction and assembly of floating offshore structures and more particularly to the construction and assembly of a spar type structure.
2. General Background
Unlike ships which can be fully assembled at an inshore facility, many types of oil drilling and production facilities for the offshore oil production industry require part of the assembly to take place either at the field location itself or at another offshore site prior to the tow to the field location. Due to the deep draft of Spar type platforms, the traditional construction sequence involves joining the structural sections of the hull in the horizontal position, transporting the completed hull in the horizontal position, followed by upending of the entire Spar to the vertical position at a site with sufficiently deep water to accommodate the deep draft.
The structural sections may consist of either plated hull tank sections only or a combination of plated tank and truss type sections. Such Spar type platforms are described in U.S. Pat. Nos. 4,702,321 and 5,558,467.
As a consequence of a horizontal assembly and transport followed by an upending sequence, numerous restrictions come into play that complicate and limit the size of the hull that can be constructed. This can result, depending on geographical location, in any or all of the following.
Draft of the assembled hull in a horizontal orientation exceeds the dredged depths in inland navigable channels for wet tow to the offshore site.
Draft of hard tank or truss sections in horizontal orientation exceeds water depths in inshore assembly areas, dry dock sill clearance depths, and/or heavy lift vessel maximum deck submergence depths. The draft restrictions imposed by fabrication facilities and transportation equipment limit the size of hulls that can be constructed.
Assembly of hull marine systems and mooring equipment in the horizontal orientation rather than the vertical operating orientation complicates fabrication, fit-up, testing and pre-commissioning of this equipment, piping and wiring.
Size and weight of hull in horizontal orientation exceeds the hydrodynamic stability and strength capabilities of the largest existing heavy lift transport vessels. This dictates transportation in sections for final horizontal assembly in an erection facility an acceptably short distance from the offshore site.
The invention addresses the above needs. What is provided is a vertical construction method. The hard tank is fabricated vertically. The hard tank is then transported in a vertical orientation to a site where it is mated to the truss section of the spar structure offshore while the hard tank and truss section are both in the vertical orientation. The mated tank and truss sections are then towed in the vertical orientation to the operational site. The hard tank is fabricated with a larger diameter and correspondingly shallower draft than a more traditionally proportioned hard tank.